It is not clear from the question that they wanted the student to “estimate” since “reasonable” does not mean “estimate”.

The “point” of Common Core math is to not teach rote methods, but have children spontaneously grok the method. The problem is that magical insight is not taught, but simply a more distant wrote method is being pushed, with the result that it is actually harder to “grok” what is going on. For example:

This is still “going through the motions” in a far more complex and indirect method. It seems to kinda make sense to those who already understand what is going on, but a kid would at best be able to go through the motions without ever groking it.